There is an inherent tendency for the retreating blade of a helicopter to stall in forward flight, which limits the forward speed of the helicopter. As helicopter rotors fly in forward flight, there is an asymmetry in lift between the advancing and retreating sides of the rotor that transmit a rolling moment to the helicopter when rigidly connected. Conventional helicopter rotors eliminate the rolling moment through the introduction of flap hinges or a gimbaled mechanism at the root of the rotor blades. The asymmetry in lift causes the blades to flap (“flapping”) which then equalizes the lift on the advancing and retreating sides. However flapping, while eliminating steady rolling moments on the airframe, also limits both the maximum speed of the helicopter, as well as its efficiency in forward flight.
Numerous techniques have been devised to increase the maximum speed and forward flight efficiency of a helicopter. One such technique uses a lift offset rotor and is shown in U.S. Pat. No. 3,409,249, by Bergquist et al., entitled “Coaxial Rigid Rotor Helicopter and Method of Flying Same.” The U.S. Pat. No. 3,409,249 describes a coaxial rigid rotor concept that introduces the idea of a lift offset rotor. The lift offset rotor was designed to minimize the potential for rotor stall by: 1) employing a first rigid rotor to inhibit the natural tendency of a rotor to equalize the lift between the advancing and retreating sides; and then by 2) balancing the net rolling moment on the helicopter through the use of a second counter-rotating rigid rotor mounted in a coaxial manner with the first rigid rotor. The term “lift offset” refers to the center of lift of the rotor and its migration toward the advancing side as airspeed is increased.
Although the coaxial rigid rotor design was successful in demonstrating the concept of avoiding retreating blade stall, the practical implementation thereof did not realize the increase in efficiency as desired. This lack of performance is a result of the loads and high vibratory moments that are generated by the design. Fundamental to the design is the use of rigid rotors that have little to no flapping associated therewith and thus introduce large fixed system vibratory moments. These moments often result in undesired airframe and rotor stresses. The airframe and rotor stresses prevent the coaxial rigid rotor design from operating with desired lift offset without significant vibration treatment. In addition, the choice of a coaxial rotor system introduces a second rotor hub that, for a light helicopter, increases the parasite drag of the aircraft by nearly 30% over a single main rotor equivalent helicopter. This increase in parasite drag negatively affects or cancels some of the efficiencies gained through the lift offset operation.
Thus, there exists a need for an improved helicopter rotor that provides lift offset, but that does not experience the loading, vibration, and other associated disadvantages as experienced with previous designs.